This invention relates to an interphase unbalance detector for AC load circuit intended to stabilize operation performance of loaded equipment, output torque for instance, by detecting the unbalance of load current of each phase when a polyphase Ac load such a three-phase induction motor is operated while controlling the supply voltage in a wide range throughout the whole area of the wide variation range at a constant rate of unbalance appropriate to the operational condition, and by cutting off the power or issuing alarm.
To keep unreeling speed of wire or film sheet(V:m/s) and winding tension(T:kg) respectively at a constant level, like in the case of winders in wire factories or roll driving units in film factories, the required output (KW) of such driving motors is obtained as a constant value of approximately T.multidot.V/102, and to the motors of these applications, a constant output is required regardless of the variation in their rate of rotation. These three-phase induction motors are called variable-speed motors and the constrained torque(Tso) at the rated voltage(Es) i.e. the torque at zero rotation(N=0) is the maximum torque, the torque(Ts) becomes zero at a synchronized number of rotation(Ns), and the intermediate torque is indicated by a specific speed-torque characteristic. If this is applied to a winder taking about one half of an ordinary rated voltage(Es/2) as the starting voltage and (2/3 Ns) as the rated number of rotation to start, the motor rotation(N) goes down from (2/3 Ns) in proportion to the increase in the wire wound around the wire drum while the wire delivered from the drawing machine at a constant speed(V) is wound around the drum. This rate of reduction is so high as 1/3 normally and 1/10 maximum. If the starting voltage(Es/2) is kept as it is, accordingly, the winding torque(Tw) increases due to the characteristic as started above, excessive tension is applied to the wire, the wire is drawn and the outside diameter goes out of the standard. The conditions for stable production of high quality wires by preventing this, therefore, are the smoothness of wide range stepless control to reduce the primary voltage of the motor corresponding to the reduction in the rate of rotation, and the adequate mutual unbalance ratio of the three-phase voltage. The unbalance of three-phase voltage cuased by trouble in the power supply system or others is multiplied becoming the unbalance of motor current, which reduces the torque and results in detective products. If one of the three phases fails, the current of the other two phases is reduced resulting in a shortage of torque as described above and defective products are made. Phase failure or unbalanced current due to malcontact of a slip ring and brush shall be detected for treatment at an early time and by each phase, if possible, otherwise the slip ring and brush may be damaged.
As described above, mutual unbalance in the load circuit causes insufficient torque and affects the quality of products, and this is applicable not only to winders as mentioned above but also to many other types of industrial equipment such as tension control rollers or cylinder driving motors in chemical textile, plastics or film production. The relays that have been used generally for motors, however, are to prevent the burning of motors, and over-current relays, 2E , 3E and 4E relays that are actuated by the over-current of other phases due to phase failure including unbalanced current have been used to prevent phase failure. In the use of the variable speed motor as mentioned above, over-current relays and other device are naturally provided to prevent burning, but so far, no adequate unbalance detector to be used on top of the above over-current relays has been found, by which unbalanced current or phase failure as the cause for the insufficient torque is detected to stabilize product quality. The static type relay such as the above 3E, for instance, can not detect a low rate of unbalance lower than 35% of a current unbalance ratio. The invention of JP. B2, No. 51-49059 as the detector for load voltage variation and unbalance in a three-phase AC circuit reduces the allowable variation range of the supply voltage when an antiphase voltage is generated and is suitable to a contact load such as rated supply voltage, but this is not applicable to a variable load as mentioned above. The short current detector for the motor circuit of JP, Y2, No. 48-44842 is to detect a shortage when the motor current goes down below a predetermined level and responds only when the short current is substantial, reduction of more than 60% of the expected value for instance, and therefore, this is not suitable for the detection of an unbalance current of a low rate either. The over-current and unbalanced current relay of JP, B2, No. 48-24311 is similar to the above 3E relay and responds to the over-current at phase failure, but the sensitivity is low at balance condition, and therefore, is not suitable for the variable speed motor as described above that generates no over-current at phase failure.
A balance relay or differential relay has been used so far to compare two or more currents or voltages for detection of unbalance and to protect generators or transformers. Among such, the moving and shading coil type proportional differential relay with three terminals which has three inhibit coils and one operation coil can detect three-phase unbalance current when used with three current transformers. However, the disadvantage is that the rotation force of the rotor becomes faint with low current reducing the detection sensitivity and making it hard to judge the phase in trouble. Moreover, the structure is more complicated than static type and the service life is shorter. To detect interphase unbalance current of the above three-phases with a static type current balance relay, on the other hand, differential current of each phase shall be applied to the level detector, and therefore, an extra one set of current transformer and full-wave rectifier are required making the circuit and other structure more complicated, and such is not suitable as a practical detector. There is a high demand, not only in the above mentioned wire industry but also in many different fields of industry, for such detectors to detect interphase unbalance of load current varying in a wide range, particularly in the low current area, at any desired high sensitivity and to prevent shortage of output torque of motors in advance, not just to protect motors against burning.
This invention, therefore, is to provide an apparatus free from such disadvantages of a conventional unbalance detector or balance relay as described above, of simple circuit structure, with free setting of the detection sensitivity of an unbalance ratio corresponding to the condition of load circuit, with constant detection sensitivity in a wide variation range of load current. Further, the apparatus can ensure stable operation such as an even outside diameter of wire products wound up by a winder for instance, by detecting the unbalance of the load during operation by each phase exactly, and by cutting off the power or by giving out alarm.